Photocatalytic structure and method for making the same

ABSTRACT

The disclosure relates to a photocatalytic structure. The photocatalytic structure includes a carbon nanotube structure, a photocatalytic active layer coated on the carbon nanotube structure, and a metal layer including a plurality of nanoparticles located on the surface of the photocatalytic active layer. The carbon nanotube structure comprises a plurality of intersected carbon nanotubes and defines a plurality of openings, and the photocatalytic active layer is coated on the surface of the plurality of carbon nanotubes. The metal layer includes a plurality of nanoparticles located on the surface of the photocatalytic active layer.

This application claims all benefits accruing under 35 U.S.C. § 119 fromChina Patent Application No. 201910075877.X, filed on Jan. 25, 2019, inthe China National Intellectual Property Administration, the disclosureof which is incorporated herein by reference. This application isrelated to applications entitled, “PHOTOCATALYTIC STRUCTURE AND METHODFOR MAKING THE SAME”, filed ______ (Atty. Docket No. US75272).

BACKGROUND 1. Technical Field

The present disclosure relates to a photocatalytic structure, and amethod for making the same.

2. Description of Related Art

The photocatalyst can produce electron-hole pairs with strongreducibility and oxidation under the irradiation of incident light whoseenergy is higher than the band gap energy of the photocatalyst. Theseelectron-hole pairs can react with substance adsorbed on the surface ofthe photocatalyst. Potential applications of photocatalysis are mainlyin the following areas: photolysis of water to produce hydrogen,artificial photosynthesis, photooxidation or decomposition of harmfulsubstances, photoelectrochemical conversion and photo-inducedsuperhydrophilicity.

As a photocatalyst, titanium dioxide has the advantages of good acidresistance, non-toxicity to organisms and large resource reserves.Titanium dioxide is a promising material for the applications in areassuch as photocatalysis and photoelectric conversion. But a large bandgap energy of titanium dioxide limits the practical applications innatural solar light. Developing forms of titanium dioxide which areresponsive to visible light is one of the most important subjects inresearch and development. Moreover, the absorption capacity of titaniumdioxide to ultraviolet light is limited, and the utilization ofultraviolet light can not reach 100%. Therefore, a lot of research anddevelopment efforts have been devoted to modifying titanium dioxide toimprove its utilization of sunlight.

Photoelectrocatalysis refers to fixing the photocatalyst on a conductivemetal. While the photocatalyst on the conductive metal acts as a workingelectrode, photogenerated electrons are forced to move toward thecounter electrode by applying a constant current or a constantpotential. As such, electrons and holes are separated.

What is needed, therefore, is an improved photocatalytic structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the embodiments. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a schematic section view of one embodiment of a photocatalyticstructure.

FIG. 2 is a cross-sectional view, along a line II-II of FIG. 1.

FIG. 3 is a flowchart of one embodiment of a method for making thephotocatalytic structure of FIG. 1.

FIG. 4 is a cross-sectional view along line IV-IV of a carbon nanotubecomposite structure of FIG. 3.

FIG. 5 is a scanning electron microscopy (SEM) image of a drawn carbonnanotube film of one embodiment.

FIG. 6 is a SEM image of an untwisted carbon nanotube wire of oneembodiment.

FIG. 7 is a SEM image of a twisted carbon nanotube wire of oneembodiment.

FIG. 8 is a SEM image of a carbon nanotube composite structure of oneembodiment.

FIG. 9 is a SEM image of a single carbon nanotube coated with an alumina(Al₂O₃) layer.

FIG. 10A is a SEM image of a substrate etched once with two intersectedstacked drawn carbon nanotube films as a mask.

FIG. 10B is a SEM image of a substrate etched once with four intersectedstacked drawn carbon nanotube films as a mask.

FIG. 10C is a SEM image of a substrate etched twice with two intersectedstacked drawn carbon nanotube films as a mask.

FIG. 11 is an Atomic Force Microscope (AFM) image of the photocatalyticstructure made by the method of FIG. 3. The carbon nanotube structureincludes two intersected stacked drawn carbon nanotube films, and thesubstrate is etched once.

FIG. 12 is an AFM image of the photocatalytic structure made by themethod of FIG. 3. The carbon nanotube structure includes two intersectedstacked drawn carbon nanotube films, and the substrate is etched twice.

FIG. 13 is an AFM image of the photocatalytic structure made by themethod of FIG. 3. The carbon nanotube structure includes fourintersected stacked drawn carbon nanotube films, and the substrate isetched once.

FIG. 14 shows diagrams of transmission vs. wavelength of visible lightof photocatalytic structures made in embodiments 1B, 1G and comparativeexamples 1-3.

FIG. 15 shows diagrams of transmission vs. wavelength of visible lightof photocatalytic structures made in embodiments 1A-1F.

FIG. 16A shows diagrams of absorbance vs. wavelength of thephotocatalytic structure of FIG. 11 for the visible light at differentlength time.

FIG. 16B shows diagrams of absorbance vs. wavelength of thephotocatalytic structure of FIG. 12 for the visible light at differentlength time.

FIG. 16C shows diagrams of absorbance vs. wavelength of thephotocatalytic structure of FIG. 13 for the visible light at differentlength time.

FIG. 17A is a Raman spectroscopy of MB molecules on the photocatalyticstructure of FIG. 11 irradiated with a 633 nm laser at different lengthtime.

FIG. 17B is a diagram of intensity of Raman peak in 1628 cm⁻¹ vs. timeat time points of FIG. 17A.

FIG. 18 is a schematic section view of another embodiment of aphotocatalytic structure.

FIG. 19 is a flowchart of another embodiment of a method for making thephotocatalytic structure of FIG. 18.

FIG. 20 is a schematic section view of another embodiment of aphotocatalytic structure.

FIG. 21 is a cross-sectional view, along a line XXI-XXI of FIG. 20.

FIG. 22 is a flowchart of another embodiment of a method for making thephotocatalytic structure of FIG. 20.

FIG. 23 is a SEM image of four intersected stacked drawn carbon nanotubefilms.

FIG. 24A is a SEM image of the photocatalytic structure with fourintersected stacked drawn carbon nanotube films as a carrier.

FIG. 24B a partial enlargement of the photocatalytic structure of FIG.24A.

FIG. 25A shows diagrams of transmission vs. wavelength ofultraviolet-visible light of photocatalytic structures made incomparative examples 4-7.

FIG. 25B shows diagrams of transmission vs. wavelength ofultraviolet-visible light of photocatalytic structures made inembodiments 3A-3D.

DETAILED DESCRIPTION

It will be appreciated that for simplicity and clarity of illustration,where appropriate, reference numerals have been repeated among thedifferent figures to indicate corresponding or analogous elements. Inaddition, numerous specific details are set forth in order to provide athorough understanding of the embodiments described herein. However, itwill be understood by those of ordinary skill in the art that theembodiments described herein can be practiced without these specificdetails. In other instances, methods, procedures, and components havenot been described in detail so as not to obscure the related relevantfeature being described. The drawings are not necessarily to scale, andthe proportions of certain parts may be exaggerated better illustratedetails and features. The description is not considered as limiting thescope of the embodiments described herein.

Several definitions that apply throughout this disclosure will now bepresented. The term “coupled” is defined as connected, whether directlyor indirectly through intervening components, and is not necessarilylimited to physical connections. The connection can be such that theobjects are permanently connected or releasably connected. The term“outside” refers to a region that is beyond the outermost confines of aphysical object. The term “inside” indicates that at least a portion ofa region is partially contained within a boundary formed by the object.The term “substantially” is defined to essentially conforming to theparticular dimension, shape or other word that substantially modifies,such that the component need not be exact. For example, substantiallycylindrical means that the object resembles a cylinder, but can have oneor more deviations from a true cylinder. The term “comprising” means“including, but not necessarily limited to”; it specifically indicatesopen-ended inclusion or membership in a so-described combination, group,series and the like. It should be noted that references to “an” or “one”embodiment in this disclosure are not necessarily to the sameembodiment, and such references mean at least one.

References will now be made to the drawings to describe, in detail,various embodiments of the present photocatalytic structure, a methodfor making the same.

Referring to FIGS. 1-2, a photocatalytic structure 10 of one embodimentis provided. The photocatalytic structure 10 comprises a substrate 12, aphotocatalytic active layer 13 on the substrate 12, and a metal layer 14on the photocatalytic active layer 13 away from the substrate 12. Thesubstrate 12 comprises a base 120 and a patterned bulge layer 122 on asurface of the base 120. The patterned bulge layer 122 is a net-likestructure comprising a plurality of strip-shaped bulges 125 intersectedwith each other. A plurality of indents 124 are defined on the base 120by the plurality of strip-shaped bulges 125. The plurality ofstrip-shaped bulges 125 is an integrated structure. The photocatalyticactive layer 13 is on the surface of the patterned bulge layer 122. Thephotocatalytic active layer 13 covers both the plurality of strip-shapedbulges 125 and the plurality of indents 124. The metal layer 14comprises a plurality of nanoparticles on the surface of thephotocatalytic active layer 13 away from the substrate 12.

The substrate 12 can be a flexible substrate or a hard substrate. Thehard substrate can be an insulative substrate, a semiconductor substrateor a metal substrate. The hard substrate can be made of a material suchas glass, quartz, silicon (Si), silicon dioxide (SiO₂), silicon nitride(Si₃N₄), gallium nitride (GaN), gallium arsenide (GaAs), alumina(Al₂O₃), magnesia (MgO), iron (Fe), copper (Cu), titanium (Ti), chromium(Cr), aluminum (Al) or zinc (Zn). When the material of the substrate 12is metal, the photocatalytic structure 10 can be used as workingelectrodes of a photoelectrocatalytic structure. The working electrodecombined with a counter electrode and a reference electrode can beassembled into a photoelectrocatalytic reactor. The photogeneratedelectrons are forced to move toward the counter electrode by applying aconstant current or a constant potential. Therefore, photogeneratedelectrons and holes are separated, which decrease recombinations ofphotogenerated charge carriers. The substrate 12 which is flexible canmake the photocatalytic structure 10 have flexibility, so that thephotocatalytic structure 10 can be attached to a curved surface.Specifically, the material of the substrate 12 can be polyethyleneterephthalate (PET), polyimide (PI), polymethyl methacrylate (PMMA),polydimethylsiloxane (PDMS), or polyethylene naphthalate (PEN), etc. Ashape, a size and a thickness of the substrate 12 are not limited andcan be selected according to applications. In one embodiment, thesubstrate 12 is a quartz wafer.

The patterned bulge layer 122 and the base 120 can be made of samematerial or different materials. In one embodiment, the patterned bulgelayer 122 and the base 120 are an integrated structure. The patternedbulge layer 122 can be located on a single surface or two oppositesurfaces of the base 120. Each of the plurality of strip-shaped bulges125 has a length less than or equal to the width or the length of thebase 120. It means that the plurality of strip-shaped bulges 125 cancover the whole surface of base 120 or part of the surface of the base120. If the 125 cover the whole surface of the base 120, the length ofthe plurality of strip-shaped bulges 125 extending along the lengthdirection is equal to the length of the base 120, and the length of theplurality of strip-shaped bulges 125 extending along the width directionis equal to the width of the base 120. If the 125 cover part of thesurface of the base 120, the length of the plurality of strip-shapedbulges 125 extending along the length direction is less than the lengthof the base 120, and the length of the plurality of strip-shaped bulges125 extending along the width direction is less than the width of thebase 120. The plurality of strip-shaped bulges 125 comprises a pluralityof first strip-shaped bulges 126 and a plurality of second strip-shapedbulges 128. The plurality of first strip-shaped bulges 126 aresubstantially parallel with each other and extends along the firstdirection, and the plurality of second strip-shaped bulges 128 aresubstantially parallel with each other and extends along the seconddirection different from the first direction. Referring to FIG. 1, thefirst direction is the ‘a’ direction and the second direction is the ‘b’direction. An angle between the first direction and the second directionis greater than 0 degrees and less than or equal to 90 degrees. In oneembodiment, the angle between the first direction and the seconddirection is greater than 30 degrees. In one embodiment, the anglebetween the first direction and the second direction is about 90degrees.

A length of each of the plurality of strip-shaped bulges 125 can beselected according to need. A width of each of the plurality ofstrip-shaped bulges 125 ranges from about 20 nanometers to about 150nanometers. In one embodiment, the width of the plurality ofstrip-shaped bulges 125 ranges from about 50 nanometers to about 100nanometers. A distance between adjacent parallel two of the plurality ofstrip-shaped bulges 125 ranges from about 50 nanometers to about 500nanometers. In one embodiment, the distance between adjacent two of theplurality of strip-shaped bulges 125 ranges from about 50 nanometers toabout 300 nanometers. In one embodiment, the distance between adjacenttwo of the plurality of strip-shaped bulges 125 ranges from about 50nanometers to about 100 nanometers. A height of the plurality ofstrip-shaped bulges 125 ranges from about 50 nanometers to about 2000nanometers. In one embodiment, the height of the plurality ofstrip-shaped bulges 125 ranges from about 300 nanometers to about 1000nanometers. The average size of the plurality of indents 124 ranges fromabout 50 nanometers to about 500 nanometers, and the depth of theplurality of indents 124 ranges from about 50 nanometers to about 2000nanometers. The size of the plurality of indents 124 refers to a maximumdiagonal length of the plurality of indents 124. In one embodiment, theratio between the depth and the average size is greater than 5. In oneembodiment, the ratio between the depth and the average size is greaterthan 10.

The photocatalytic active layer 13 can be located on both top and sidesurfaces of the plurality of strip-shaped bulges 125 and bottom surfacesof the plurality of indents 124. The material of photocatalytic activelayer 13 can be titanium dioxide (TiO₂), zinc oxide (ZnO), tin oxide(SnO₂), zirconium dioxide (ZrO₂), cadmium sulfide (CdS) and other oxideor sulfide semiconductors. The material of the photocatalytic activelayer 13 comprises at least one of the above materials. The titaniumdioxide is available in three crystal structures-anatase, rutile andslate. Only the anatase titanium dioxide and rutile titanium dioxidehave photocatalytic activity. In one embodiment, the material of thephotocatalytic active layer 13 is anatase titanium dioxide.

The metal layer 14 is located on the surface of the photocatalyticactive layer 13 away from the substrate 12. The metal layer 14 can be acontinuous structure and cover the entire surface of the photocatalyticactive layer 13. The metal layer 14 can also be a discontinuousstructure. The metal layer 14 can be a single-layer or a multi-layerstructure. Surface plasmon resonance exists on the surface of metallayer 14 at the indents 124 to enhance the absorption of visible light.The localized surface plasmon resonance effect produced by the metallayer 14 can make the excited thermal electrons cross the conductionband of the photocatalytic active layer 13, inhibit the recombinationprobability of photogenerated electron-hole pairs in the photocatalyticactive layer 13, and increase the number of free electrons in theconduction band of the photocatalytic active layer 13.

The thickness of the metal layer 14 ranges from about 2 nanometers toabout 200 nanometers. The material of the metal layer 14 is a metalmaterial with surface plasmon effect. The material of the metal layer 14can be gold, silver, copper, iron, nickel, aluminum, or any alloythereof. The metal layer 14 can be uniformly deposited on the surface ofthe photocatalytic active layer 13 by a method of electron beamevaporation, chemical vapor deposition (CVD), or sputtering. In oneembodiment, the metal layer 14 is a gold layer with a thickness of about8 nanometers.

Referring to FIG. 3 and FIG. 4, a method for making the photocatalyticstructure 10 of one embodiment comprises the following steps:

step (S10), providing a substrate 12;

step (S20), providing a carbon nanotube composite structure 110, whereinthe carbon nanotube composite structure 110 includes a plurality ofintersected carbon nanotubes and defines a plurality of openings 116;

step (S30), placing the carbon nanotube composite structure 110 on asurface 121 of the substrate 12, wherein parts of the surface 121 areexposed from the plurality of openings 116;

step (S40), forming the patterned bulge layer 122 on the surface 121 bydry etching the surface 121 using the carbon nanotube compositestructure 110 as a mask, wherein the patterned bulge layer 122 includesa plurality of strip-shaped bulges 125 intersected with each other;

step (S50), removing the carbon nanotube composite structure 110;

step (S60), applying a photocatalytic active layer 13 on the patternedbulge layer 122;

step (S70), applying a metal layer pre-form 15 on the surface of thephotocatalytic active layer 13 away from the substrate 12; and

step (S80), forming the metal layer 14 by annealing the metal layerpre-form 15.

In step (S10), the material of the substrate 12 is not limited and canbe metal, insulating material or semiconductor. The metal can be gold,aluminum, nickel, chromium, or copper. The insulating material can besilicon dioxide or silicon nitride. The semiconductor can be silicon,gallium nitride, or gallium arsenide. In one embodiment, the material ofthe substrate 12 is a quartz wafer with a thickness of 500 micrometers.

In step (S20), the carbon nanotube composite structure 110 includes acarbon nanotube structure 112 and a protective layer 114 coated on thecarbon nanotube structure 112 as shown in FIG. 4. The carbon nanotubestructure 112 is a free-standing structure. The term “free-standingstructure” includes that the carbon nanotube structure 112 can sustainthe weight of itself when it is hoisted by a portion thereof without anysignificant damage to its structural integrity. Thus, the carbonnanotube structure 112 can be suspended by two spaced supports.

The plurality of carbon nanotubes can be single-walled carbon nanotubes,double-walled carbon nanotubes, or multi-walled carbon nanotubes. Thelength and diameter of the plurality of carbon nanotubes can be selectedaccording to need. The diameter of the single-walled carbon nanotubesranges from about 0.5 nanometers to about 10 nanometers. The diameter ofthe double-walled carbon nanotubes ranges from about 1.0 nanometer toabout 15 nanometers. The diameter of the multi-walled carbon nanotubesranges from about 1.5 nanometers to about 50 nanometers. A length of thecarbon nanotubes is larger than 50 micrometers. In one embodiment, thelength of the carbon nanotubes ranges from about 200 micrometers toabout 900 micrometers.

The plurality of carbon nanotubes are orderly arranged to form anordered carbon nanotube structure. The plurality of carbon nanotubesextend along a direction substantially parallel to the surface of thecarbon nanotube structure 112. The largest surface of the carbonnanotube structure 112 is formed by arranging the plurality of carbonanotubes substantially parallel in the surface. The term ‘orderedcarbon nanotube structure’ includes, but is not limited to, a structurewherein the plurality of carbon nanotubes are arranged in a consistentlysystematic manner, e.g., the plurality of carbon nanotubes are arrangedapproximately along the same direction.

The carbon nanotube structure 112 defines a plurality of apertures. Theaperture penetrates through the thickness direction of the carbonnanotube structure 112. The aperture can be a hole defined by severaladjacent carbon nanotubes, or a gap defined by two substantiallyparallel carbon nanotubes and extending along axial direction of thecarbon nanotubes. The hole shaped aperture and the gap shaped aperturecan co-exist in the carbon nanotube structure 112. Hereafter, the sizeof the aperture is a maximum diagonal length of the hole or width of thegap. The sizes of the apertures can be different. The average size ofthe apertures ranges from about 10 nanometers to about 500 micrometers.For example, the sizes of the apertures can be about 50 nanometers, 100nanometers, 500 nanometers, 1 micrometer, 10 micrometers, 80micrometers, or 120 micrometers.

The carbon nanotube structure 112 can include at least one carbonnanotube film, at least one carbon nanotube wire, or combinationthereof. In one embodiment, the carbon nanotube structure 112 caninclude a single carbon nanotube film or two or more carbon nanotubefilms stacked together. Thus, the thickness of the carbon nanotubestructure 112 can be controlled by the number of the stacked carbonnanotube films. The number of the stacked carbon nanotube films rangesfrom about 2 to about 100. For example, the number of the stacked carbonnanotube films can be 2, 3, or 4. In one embodiment, the carbon nanotubestructure 112 is formed by folding a single carbon nanotube wire. In oneembodiment, the carbon nanotube structure 112 can include a layer ofparallel and spaced carbon nanotube wires. Also, the carbon nanotubestructure 112 can include a plurality of carbon nanotube wiresintersected or weaved together to form a carbon nanotube net. Thedistance between two adjacent parallel and spaced carbon nanotube wiresranges from about 0.1 micrometers to about 200 micrometers. In oneembodiment, the distance between two adjacent parallel and spaced carbonnanotube wires is in a range from about 10 micrometers to about 100micrometers. The gap between two adjacent substantially parallel carbonnanotube wires is defined as the apertures. The size of the aperturescan be controlled by controlling the distance between two adjacentparallel and spaced carbon nanotube wires. The length of the gap betweentwo adjacent parallel carbon nanotube wires can be equal to the lengthof the carbon nanotube wire. It is understood that any carbon nanotubestructure described can be used with all embodiments.

In one embodiment, the carbon nanotube structure 112 includes at leastone drawn carbon nanotube film. For example, the number of the drawncarbon nanotube films can be 2, 3, or 4. The drawn carbon nanotube filmcan be drawn from a carbon nanotube array that is able to have a filmdrawn therefrom. The drawn carbon nanotube film includes a plurality ofsuccessive and oriented carbon nanotubes joined end-to-end by van derWaals attractive force therebetween. The drawn carbon nanotube film is afree-standing film. Referring to FIG. 5, each drawn carbon nanotube filmincludes a plurality of successively oriented carbon nanotube segmentsjoined end-to-end by van der Waals attractive force therebetween. Eachcarbon nanotube segment includes a plurality of carbon nanotubesparallel to each other, and combined by van der Waals attractive forcetherebetween. As can be seen in FIG. 5, some variations can occur in thedrawn carbon nanotube film. The carbon nanotubes in the drawn carbonnanotube film are oriented along a preferred orientation. The drawncarbon nanotube film can be treated with an organic solvent to increasethe mechanical strength and toughness and reduce the coefficient offriction of the drawn carbon nanotube film. A thickness of the drawncarbon nanotube film can range from about 0.5 nanometers to about 100micrometers. The drawn carbon nanotube film defines a plurality ofapertures between adjacent carbon nanotubes.

The carbon nanotube structure 112 can include at least two stacked drawncarbon nanotube films. In other embodiments, the carbon nanotubestructure 112 can include two or more coplanar carbon nanotube films,and can include layers of coplanar carbon nanotube films. Additionally,when the carbon nanotubes in the carbon nanotube film are aligned alongone preferred orientation (e.g., the drawn carbon nanotube film), anangle can exist between the orientation of carbon nanotubes in adjacentfilms, whether stacked or adjacent. Adjacent carbon nanotube films canbe combined by only the van der Waals attractive force therebetween. Anangle between the aligned directions of the carbon nanotubes in twoadjacent carbon nanotube films can range from about 0 degrees to about90 degrees. When the angle between the aligned directions of the carbonnanotubes in adjacent stacked drawn carbon nanotube films is larger than0 degrees, a plurality of micro pores is defined by the carbon nanotubestructure 112. In one embodiment, the carbon nanotube structure 112 hasthe aligned directions of the carbon nanotubes between adjacent stackeddrawn carbon nanotube films at 90 degrees. Stacking the carbon nanotubefilms will also add to the structural integrity of the carbon nanotubestructure 112.

The carbon nanotube wire can be untwisted or twisted. Treating the drawncarbon nanotube film with a volatile organic solvent can form theuntwisted carbon nanotube wire. Specifically, the organic solvent isapplied to soak the entire surface of the drawn carbon nanotube film.During the soaking, adjacent parallel carbon nanotubes in the drawncarbon nanotube film will bundle together, due to the surface tension ofthe organic solvent as it volatilizes, and thus, the drawn carbonnanotube film will be shrunk into an untwisted carbon nanotube wire.Referring to FIG. 6, the untwisted carbon nanotube wire includes aplurality of carbon nanotubes substantially oriented along the samedirection (i.e., a direction along the length of the untwisted carbonnanotube wire). The carbon nanotubes are substantially parallel to theaxis of the untwisted carbon nanotube wire. More specifically, theuntwisted carbon nanotube wire includes a plurality of successive carbonnanotube segments joined end to end by van der Waals attractive forcetherebetween. Each carbon nanotube segment includes a plurality ofcarbon nanotubes substantially parallel to each other, and combined byvan der Waals attractive force therebetween. The carbon nanotubesegments can vary in width, thickness, uniformity, and shape. The lengthof the untwisted carbon nanotube wire can be arbitrarily set as desired.A diameter of the untwisted carbon nanotube wire ranges from about 0.5nanometers to about 100 micrometers.

The twisted carbon nanotube wire can be formed by twisting a drawncarbon nanotube film using a mechanical force to turn the two ends ofthe drawn carbon nanotube film in opposite directions. Referring to FIG.7, the twisted carbon nanotube wire includes a plurality of carbonnanotubes helically oriented around an axial direction of the twistedcarbon nanotube wire. More specifically, the twisted carbon nanotubewire includes a plurality of successive carbon nanotube segments joinedend to end by van der Waals attractive force therebetween. Each carbonnanotube segment includes a plurality of carbon nanotubes parallel toeach other, and combined by van der Waals attractive force therebetween.The length of the carbon nanotube wire can be set as desired. A diameterof the twisted carbon nanotube wire can be from about 0.5 nanometers toabout 100 micrometers. Further, the twisted carbon nanotube wire can betreated with a volatile organic solvent after being twisted to bundlethe adjacent paralleled carbon nanotubes together. The specific surfacearea of the twisted carbon nanotube wire will decrease, while thedensity and strength of the twisted carbon nanotube wire will increase.

The carbon nanotube composite structure 110 can be made by applying aprotective layer 114 on a surface of the carbon nanotube structure 112.The carbon nanotube structure 112 can be suspended in a depositingchamber during depositing the protective layer 114 so that two oppositesurfaces of the carbon nanotube structure 112 are coated with theprotective layer 114. In some embodiments, each of the plurality ofcarbon nanotubes is fully enclosed by the protective layer 114. In oneembodiment, the carbon nanotube structure 112 is located on a frame sothat the middle portion of the carbon nanotube structure 112 issuspended through the through hole of the frame. The frame can be anyshape, such as a quadrilateral. The carbon nanotube structure 112 canalso be suspended by a metal mesh or metal ring.

The method of depositing the protective layer 114 can be physical vapordeposition (PVD), chemical vapor deposition (CVD), atomic layerdeposition (ALD), magnetron sputtering, or spraying.

The plurality of openings 116 are formed from the plurality of aperturesof the carbon nanotube structure 112. The plurality of openings 116 havea shape and a size corresponding to the plurality of apertures. The sizeof the plurality of openings 116 is smaller than that of the pluralityof apertures because the protective layer 114 is deposited in theplurality of apertures.

The thickness of the protective layer 114 is in a range from about 3nanometers to about 50 nanometers. In one embodiment, the thickness ofthe protective layer 114 is in a range from about 3 nanometers to about20 nanometers. If the thickness of the protective layer 114 is less than3 nanometers, the protective layer 114 cannot prevent the carbonnanotubes from being destroyed in following etching process. If thethickness of the protective layer 114 is greater than 50 nanometers, theplurality of apertures may be fully filled by the protective layer 114and the plurality of openings 116 cannot be obtained.

The material of the protective layer 114 can be metal, metal oxide,metal nitride, metal carbide, metal sulfide, silicon oxide, siliconnitride, or silicon carbide. The metal can be gold, nickel, titanium,iron, aluminum, titanium, chromium, or alloy thereof. The metal oxidecan be alumina, magnesium oxide, zinc oxide, or hafnium oxide. Thematerial of the protective layer 114 is not limited above and can be anymaterial as long as the material can be deposited on the carbon nanotubestructure 112, would not react with the carbon nanotubes and would notbe etched easily in following drying etching process. The protectivelayer 114 is combined with the carbon nanotube structure 112 by van derWaals attractive force therebetween only.

As shown in FIG. 8, in one embodiment, an alumina layer of 20 nanometersthickness is deposited on two stacked drawn carbon nanotube films byelectron beam evaporation. As shown in FIG. 9, each of the carbonnanotubes is entirely coated by the alumina layer. The aligned directionof the carbon nanotubes between adjacent stacked drawn carbon nanotubefilms is 90 degrees.

In step (S30), the carbon nanotube composite structure 110 can be indirect contact with the surface 121 of the substrate 12 or suspendedabove the surface 121 of the substrate 12 by a support. In oneembodiment, the carbon nanotube composite structure 110 is transferredon the surface 121 of the substrate 12 through the frame.

In one embodiment, the placing the carbon nanotube composite structure110 on the surface 121 further comprises solvent treating the substrate12 with the carbon nanotube composite structure 110 thereon. Because airis entrapped between the carbon nanotube composite structure 110 and thesurface 121 of the substrate 12, the solvent treating can exhaust theair and allow the carbon nanotube composite structure 110 to be closelyand firmly adhered on the surface 121 of the substrate 12. The solventtreating can be applying a solvent to entire surface of the carbonnanotube composite structure 110 or immersing the entire substrate 12with the carbon nanotube composite structure 110 in a solvent. Thesolvent can be water or volatile organic solvent such as ethanol,methanol, acetone, dichloroethane, chloroform, or mixtures thereof. Inone embodiment, the organic solvent is ethanol.

In step (S40), the dry etching can be plasma etching or reactive ionetching (RIE). In one embodiment, the dry etching is performed byapplying plasma energy on the entire or part surface of the surface 121via a plasma device. The plasma gas can be an inert gas and/or etchinggases, such as argon (Ar), helium (He), chlorine (Cl₂), hydrogen (H₂),oxygen (O₂), fluorocarbon (CF₄), ammonia (NH₃), or air.

In one embodiment, the plasma gas is fluorocarbon. The power of theplasma device ranges from about 20 watts to about 70 watts. The plasmaflow of fluorocarbon ranges from about 10 sccm to about 60 sccm, such as40 sccm. When the plasma is produced in vacuum, the work pressure of theplasma ranges from about 1 Pa to 10 Pa, such as 2 Pa. The time forplasma etching ranges from about 20 seconds to about 300 seconds, suchas 200 seconds.

In the plasma etching process, the plasma gas would react with theexposed portion of the substrate 12 and would not react with theprotective layer 114, or reaction between the plasma gas and theprotective layer 114 is much slower than reaction between the plasma gasand the substrate 12. The selection relationship of the plasma gas,material of the substrate 12 and material of the protective layer 114 isshown in Table 1 below.

TABLE 1 Number Substrate Protective layer Plasma gas 1 Al SiO₂ Cl₂ orBCl₃ 2 SiO₂ Al, Cr, Fe, Ti, Ni, Au or Al₂O₃ CF₄ 3 SiN_(x) Al, Cr, Fe,Ti, Ni, or Au CF₄ 4 GaN Al₂O₃ Cl₂ or Ar₂ 5 Au•Cr or Ni SiO₂, or SiN_(x)O₂ or Ar₂ 6 Cu SiO₂, or SiN_(x) O₂ or BCl₃

In the etching process, the etching gas reacts with the substrate 12,but does not react with the protective layer 114 or react with theprotective layer 114 at a speed much less than that of the reactionbetween the etching gas and the substrate 12. Thus, the exposed portionof the substrate 12 would be etched gradually and the portion of thesubstrate 12 that are shielded by the carbon nanotube compositestructure 110 would not be etched.

The patterned bulge layer 122 and the carbon nanotube compositestructure 110 substantially have the same pattern. When the carbonnanotube structure 112 includes a plurality of intersected drawn carbonnanotube films, the patterned bulge layer 122 includes a plurality ofstrip-shaped bulges 125 intersected with each other to form a netstructure as shown in FIG. 11 to FIG. 13. An angle between the aligneddirections of the carbon nanotubes in two adjacent stacked drawn carbonnanotube films is less than 0 degrees and larger than or equal to 90degrees. The aligned directions of the carbon nanotubes in any two drawncarbon nanotube films are different. In one embodiment, the carbonnanotube structure 112 includes two intersected stacked drawn carbonnanotube films. The angle between the aligned directions of the carbonnanotubes in the two adjacent stacked drawn carbon nanotube films isabout 90 degrees. In another embodiment, the carbon nanotube structure112 includes four intersected stacked drawn carbon nanotube films. Theangle between the aligned directions of the carbon nanotubes in any twoadjacent stacked drawn carbon nanotube films is about 45 degrees and thealigned directions of the carbon nanotubes in any two drawn carbonnanotube films are different from each other.

The inventors of instant application find that the photocatalyticstructure prepared by different intersected drawn carbon nanotube filmsas masks can adjust the absorption of the visible light. Thenanostructures on the surface of the substrates obtained by dry etchingwith different intersected stacked drawn carbon nanotube films as masksare different. FIG. 10A shows a SEM image of a substrate etched oncewith two intersected stacked drawn carbon nanotube films as a mask. FIG.10B shows a SEM image of a substrate etched once with four intersectedstacked drawn carbon nanotube films as a mask. FIG. 10C shows a SEMimage of a substrate etched twice with two intersected stacked drawncarbon nanotube films as a mask. Before the second etching, the carbonnanotube composite structure 110 rotates around the central axisperpendicular to the surface of the drawn carbon nanotube films. Arotation angle is greater than 0 degrees and less than 90 degrees. Theplurality of strip-shaped bulges obtained by the first etching arebroken after the second etching with the rotated carbon nanotubecomposite structure 110 as a mask, so that smaller and morenanostructures are formed on the surface of the substrate 12. The layersand the cross manner of the stacked drawn carbon nanotube films canaffect the nanostructures on the surface of the substrates obtained bydry etching. Even if the layers are the same, different etching modescan also make different nanostructures on the surface of substrate 12.The more layers of the intersected drawn carbon nanotube films, thesmaller size of the nanostructures on the surface of substrate 12.

The plurality of strip-shaped bulges 125 can have a width in a rangefrom about 20 nanometers to about 150 nanometers, a distance in a rangefrom about 50 nanometers to about 500 nanometers, and a height in arange from about 50 nanometers to about 1000 nanometers. In oneembodiment, the plurality of strip-shaped bulges 125 have a width in arange from about 50 nanometers to about 100 nanometers, a distance in arange from about 50 nanometers to about 100 nanometers, and a height ina range from about 300 nanometers to about 1000 nanometers.

After coating with the protective layer 114, the diameter of the carbonnanotubes are about tens of nanometers, and distance between adjacenttwo carbon nanotubes are about tens of nanometers. Thus, the width anddistance of the plurality of strip-shaped bulges 125 are also tens ofnanometers, and the average size of the plurality of indents 124 arealso tens of nanometers. The density of the strip-shaped bulges 125 andthe indents 124 would be increased. For example, when both the width anddistance of the plurality of strip-shaped bulges 125 are 50 nanometers,the number of the strip-shaped bulges 125 and the indents 124 would be20 within 1 micrometer. The conventional photolithography method cannotmake all the strip-shaped bulges in nano-scale and obtain this densitydue to the resolution limitation. The nanostructures can increase thespecific surface area of the photocatalytic structure 10 so that thecontact area between the photocatalytic structure 10 and sunlight willbe enhanced, and the amount of the pollutants adsorbed on thephotocatalytic structure 10 will be also increased. If the sunlightirradiates on the nanostructures of the photocatalytic structure 10,part of the sunlight can be absorbed by the nanostructures, and part ofthe sunlight can be reflected by the nanostructures. The sunlightreflected by the nanostructures can irradiate on the adjacentnanostructures, and a part of the reflected sunlight can be absorbed bythe nanostructures. Therefore, the sunlight irradiating on thenanostructures can be reflected and absorbed many times by thenanostructures. Thus, the light utilization efficiency of photocatalyticstructure 10 can be improved.

In step (S50), the method of removing the carbon nanotube compositestructure 110 can be ultrasonic method, or adhesive tape peeling,oxidation. In one embodiment, the substrate 12 with the carbon nanotubecomposite structure 110 thereon is placed in an N-methyl pyrrolidonesolution and ultrasonic vibrated for several minutes.

In step (S60), the photocatalytic active layer 13 can be deposited onboth top and side surfaces of the plurality of strip-shaped bulges 125and bottom surfaces of the plurality of indents 124. The photocatalyticactive layer 13 can be formed by depositing titanium dioxide directly.The photocatalytic active layer 13 can also be formed by puttering ordeposition of titanium in oxygen flow. In one embodiment, the method offorming the photocatalytic active layer 13 is atomic layer deposition(ALD). The photocatalytic structure 10 made by atomic layer depositionhas higher photocatalytic efficiency because the atomic layer depositionmethod deposits a titanium dioxide layer with a uniform thickness and abetter coverage of the substrate. Furthermore, the deposited titaniumdioxide is anatase structure because the titanium dioxide is formed anddeposited at a high temperature according to the present embodiment. Thetitanium dioxide layer can also be obtained by other methods, such asevaporation and sputtering, but annealing of the titanium dioxide layerin the air is needed to transform the titanium dioxide into anatasestructure. In one embodiment, the thickness of the titanium dioxidelayer is in a range from about 2 nanometers to about 600 nanometers. Inanother embodiment, the thickness of the titanium dioxide layer is in arange from about 15 nanometers to about 300 nanometers. In anotherembodiment, the thickness of the titanium dioxide layer ranges fromabout 20 nanometers to about 80 nanometers.

In step (S70), the metal layer pre-form 15 can be deposited on thephotocatalytic active layer 13 by a method of electron beam evaporation,ion beam sputtering, atomic layer deposition, magnetron sputtering,thermal vapor deposition, or chemical vapor deposition. The thickness ofthe metal layer pre-form 15 ranges from about 2 nanometers to about 100nanometers. The thickness of the metal layer pre-form 15 can also be ina range from about 5 nanometers to about 20 nanometers. In oneembodiment, the thickness of the metal layer pre-form 15 is about 8nanometers. The material of the metal layer pre-form 15 can be gold,silver, copper, iron, nickel, aluminum or alloy thereof. In oneembodiment, the metal layer pre-form 15 is a gold layer with a thicknessof about 8 nanometers. In one embodiment, the gold layer covers entiresurfaces of the photocatalytic active layer 13.

In step (S80), the annealing is carried out in a protective gasenvironment. The protective gas can be nitrogen, or inert gas such asargon or the like. The annealing temperature ranges from about 200° C.to about 1000° C. In one embodiment, the annealing temperature is in arange from about 500° C. to about 800° C. The annealing time ranges fromabout 10 minutes to about 6 hours. In one embodiment, the annealing timeranges from about 20 minutes to about 3 hours. In another embodiment,the annealing time ranges from about 30 minutes to about 50 minutes.After annealing, the metal pre-form layer 15 on the surface of thephotocatalytic active layer 13 is transformed to a plurality ofnanoparticles.

In order to research the effect of the layer number of the intersectedstacked drawn carbon nanotube films, the deposited metal, and theannealing treatment on the property of the photocatalytic structure, thefollowing experiments on embodiments 1A to 1G were carried out. Forreferencing purposes, experiments on comparative example 1 tocomparative example 3 were carried out under the same controllingconditions.

TABLE 2 Different conditions for making photocatalytic structure inembodiments 1A to 1G and comparative examples 1 to 3 Layer number ofType of intersected stacked drawn Echting Type of Annealing Embodimentssubstrate carbon nanotube films time deposited metals or not 1AStructure 2 1 Au No 1B Structure 2 1 Au Yes 1C Structure 2 2 Au No 1DStructure 2 2 Au Yes 1E Structure 4 1 Au No 1F Structure 4 1 Au Yes 1GStructure 2 1 Ag Yes Comparative Pure 0 0 No deposited Yes example 1metal Comparative Structure 2 1 No deposited Yes example 2 metalComparative Pure 0 0 Au Yes example 3

The structured substrate refers that the substrate 12 comprises a base120 and a patterned bulge layer 122 on a surface of the base 120. Thepure substrate refers that the substrate 12 has no patterned bulge layer122 on a surface of the base 120. Referring to FIG. 11-FIG. 13, atomicforce microscope (AFM) images of the photocatalytic structure made byembodiment 1B, embodiment 1D, and embodiment 1F are provided. Thetransmission spectra of visible light for the photocatalytic structuresprepared in embodiments 1A-1G and comparative examples 1-3 is alsoprovided. Referring to FIG. 14, it is found that the visible lighttransmission of the photocatalytic structures with the patterned bulgelayer is generally lower than that of photocatalytic structures withoutany patterned bulge layer. The visible light absorbance of thephotocatalytic structures with the patterned bulge layer is higher thanthat of the photocatalytic structures without any patterned bulge layer,because the patterned bulge layer 122 comprises a plurality ofstrip-shaped bulges intersected with each other. The spacing between theadjacent strip-shaped bulges can be adjusted, so that the resonancewavelength can be adjusted and the absorption of visible light can beimproved.

Referring to FIG. 15, the visible light absorbance of the photocatalyticstructure after annealing is higher than that of the photocatalyticstructure without annealing treatment. Metal nanoparticles will beformed on the surface of the photocatalytic active layer 13 afterannealing the metal layer pre-form 15. Because of the localized surfaceplasmon resonance (LSPR), electric filed intensity on the surface of themetal nanoparticles is increased. The localized surface plasmonresonance of the metal nanoparticles can be formed at certainwavelengths, so that the metal nanoparticles can achieve broadbandabsorption.

The inventors of instant application find that the photocatalyticstructure prepared by different intersected drawn carbon nanotube filmsas masks can adjust the absorption of the visible light. Thenanostructures on the surface of the substrates obtained by dry etchingwith different intersected drawn carbon nanotube films as masks aredifferent. The photocatalytic structure with the substrate etched oncewith two intersected stacked drawn carbon nanotube films as a mask haslittle difference in visible light absorption from that of thephotocatalytic structure with substrate etched twice with twointersected stacked drawn carbon nanotube films as a mask. The maximumabsorbance of visible light occurs at 633 nanometer wavelength. Themaximum absorbance of visible light of the photocatalytic structure withsubstrate etched once with four intersected stacked drawn carbonnanotube films as a mask occurs at 660 nanometer wavelength. Theabsorbance can be 67%. The layers and the cross manner of the stackeddrawn carbon nanotube films can affect the nanostructures on the surfaceof the substrates obtained by dry etching, so that the visible lightabsorbance of the photocatalytic structure 10 in a specific band can bemaximized. It is not that the more the layers of the stacked drawncarbon nanotube films, the better the property of the photocatalyticstructure. When the layers are more than 4, the substrate can not beetched easily because more layers make the micro pore size smaller. Thephotocatalytic effect of the photocatalytic structure prepared by morethan four intersected stacked drawn carbon nanotube films as a maskbecomes worse than the photocatalytic effect of the photocatalyticstructure prepared by four intersected stacked drawn carbon nanotubefilms as a mask.

The performance of photocatalytic structures prepared by embodiments 1B,1D and 1F was tested with methylene blue (MB). 10 mM MB is spin coatedon a surface of metal layer 14 of the photocatalytic structure 10,wherein the rotation speed can be about 2000 rpm, and the spin time canbe about 60 seconds. Simulated sunlight is supplied to the MB to producean absorption spectrum. Referring to FIG. 16, the intensity ofcharacteristic peaks of MB molecules was reduced after simulatedsunlight irradiation of 45 minutes, which indicated that thephotocatalytic structures prepared by embodiments 1B, 1D and 1F had highcatalytic effect under simulated sunlight, and the utilization ofvisible light is improved. Referring to FIG. 17A and FIG. 17B, Ramanspectroscopies of MB molecules is provided. A laser irradiation of 633nanometers is supplied to the MB molecules by a Raman detection systemto produce a Raman spectroscopy. The intensity of characteristic peaksof MB molecules was reduced over time, which indicates that the amountof MB molecules decreased as the MB molecules decompose under visiblelaser irradiation. FIG. 17B is a diagram of the intensity of Raman peakin 1628 cm⁻¹ of MB molecules over time. The intensity of the Raman peakin 1628 cm⁻¹ was reduced, which demonstrates that photocatalyticstructure has good photocatalytic effect under 633 nanometers visiblelight irradiation. The photocatalytic structure can improve thephotocatalytic response and utilization under visible light.

The photocatalytic structure 10 as disclosed has the followingcharacteristics. Firstly, the photocatalytic active layer 13 is locatedon the surface of patterned bulge layer 122 comprising a plurality ofstrip-shaped bulges intersected with each other, so the specific surfacearea can be increased and the adsorption capacity of the reactants canbe enhanced. Secondly, metal surface plasma resonance absorption willoccur under the excitation of incident photoelectric magnetic field. Thethermal electrons generated by the metal layer 14 due to the localsurface plasmon resonance effect can be released onto the conductionband of the photocatalytic active layer 13. Surface plasmon resonanceeffect can inhibit the recombination rate of electron-hole pairs andincrease the number of active electrons through the transfer of energyor charge carriers. The absorbance and utilization of light can beimproved. Thirdly, the nanostructure of photocatalytic structure 10 andthe resonance absorption wavelength and absorption intensity of themetal layer 14 to visible light can be controlled by adjusting the layernumber and the intersecting and stacking manners of the drawn carbonnanotube films in carbon nanotube structure 112. Experiment resultsaccording to embodiments of the present application show that theabsorbance of visible light and the catalytic effect of titanium dioxidein certain wavelength can be greatly improved.

Referring to FIG. 18, a photocatalytic structure 10B of anotherembodiment is disclosed. The photocatalytic structure 10B comprises asubstrate 12, a carbon nanotube composite structure 110, aphotocatalytic active layer 13, and a metal layer 14. The substrate 12comprises a base 120 and a patterned bulge layer 122 on a surface of thebase 120. The patterned bulge layer 122 is a net-like structurecomprising a plurality of strip-shaped bulges 125 intersected with eachother and a plurality of indents 124 defined by the plurality ofstrip-shaped bulges 125. The plurality of strip-shaped bulges 125 is anintegrated structure. The carbon nanotube composite structure 110 islocated between the patterned bulge layer 122 and the photocatalyticactive layer 13.

The photocatalytic structure 10B is similar to the photocatalyticstructure 10 above except that the photocatalytic structure 10B furthercomprises a carbon nanotube composite structure 110 located between thepatterned bulge layer 122 and the photocatalytic active layer 13. Thephotocatalytic active layer 13 entirely covers both the patterned bulgelayer 122 and the carbon nanotube composite structure 110. The carbonnanotube composite structure 110 is on the top surface of the pluralityof strip-shaped bulges 125 opposite to the plurality of indents 124.

Referring to FIG. 19, a method for making the photocatalytic structure10B of one embodiment includes the following steps:

step (S10B), placing the carbon nanotube composite structure 110 on asurface 121 of the substrate 12, wherein parts of the surface 121 areexposed from the plurality of openings 116;

step (S20B), forming the patterned bulge layer 122 on the surface 121 bydry etching the surface 121 using the carbon nanotube compositestructure 110 as a mask, wherein the patterned bulge layer 122 includesa plurality of strip-shaped bulges 125 intersected with each other;

step (S30B), applying the photocatalytic active layer 13 on thepatterned bulge layer 122 so that the photocatalytic active layer 13entirely covers both the patterned bulge layer 122 and the carbonnanotube composite structure 110;

step (S40B), applying a metal layer pre-form 15 on the surface of thephotocatalytic active layer 13 away from the substrate 12; and

step (S50B), forming the metal layer 14 by annealing the metal layerpre-form 15.

The method for making the photocatalytic structure 10B is similar to themethod for making the photocatalytic structure 10 above except the step(S50) is omitted.

The carbon nanotube composite structure 110 and the patterned bulgelayer 122 can form two layer of nano-scaled structure having the samepattern. The carbon nanotube composite structure 110 can further enhancethe roughness of the top surfaces of the patterned bulge layer 122.Thus, the absorbance of the light and the photocatalytic effect will befurther improved. Furthermore, the method for making the photocatalyticstructure 10B would have a relatively lower cost and relatively higherefficiency, and cause less pollution because the step (S50) of removingthe carbon nanotube composite structure 110 is omitted.

Referring to FIG. 20 and FIG. 21, a photocatalytic structure 10C ofanother embodiment is provided. The photocatalytic structure 10Ccomprises a carbon nanotube structure 112, a photocatalytic active layer13 coated on the carbon nanotube structure 112, and a metal layer 14coated on the photocatalytic active layer 13. The carbon nanotubestructure 112 comprises a plurality of intersected carbon nanotubes anddefines a plurality of openings 116. The photocatalytic active layer 13is coated on the surface of the plurality of carbon nanotubes.

The metal layer 14 comprises a plurality of nanoparticles on the surfaceof the photocatalytic active layer 13. The thickness of thephotocatalytic structure 10C is micro-nano meters scale. The sizes ofthe plurality of openings are also micro-nano meters. The size of theplurality of openings is a maximum diagonal length of the hole or widthof the gap. In one embodiment, the thickness of the photocatalyticstructure 10C is in a range from 50 nanometers to 300 micrometers, andthe sizes of the plurality of openings 116 are in a range from 1nanometer to 0.5 micrometers.

Referring to FIG. 22, a method for making the photocatalytic structure10C of one embodiment includes the following steps:

step (S10C), providing a carbon nanotube structure 112, wherein thecarbon nanotube structure 112 includes a plurality of intersected carbonnanotubes and defines a plurality of openings 116

step (S20C), forming the photocatalytic active layer 13 on the surfaceof the carbon nanotube structure 112;

step (S30C), applying a metal layer pre-form 15 on the surface of thephotocatalytic active layer 13; and

step (S40C), forming the metal layer 14 by annealing the metal layerpre-form 15.

In step (S10C), the carbon nanotube structure 112 comprises at least twointersected stacked drawn carbon nanotube films. The alignment directionof carbon nanotubes in any two carbon nanotube films has an angle of β,and the angle β is greater than 0 degrees and less than or equal to 90degrees. Therefore, a plurality of opening 116 is defined. The sizes ofthe plurality of opening 116 range from 1 nanometer to 0.5 micrometers.

In step (20C), the thickness of the photocatalytic active layer 13ranges from about 2 nanometers to about 600 nanometers. In oneembodiment, the thickness of the photocatalytic active layer 13 rangesfrom about 25 nanometers to about 100 nanometers.

Two methods of forming the photocatalytic active layer 13 are disclosedas follows:

One of the methods of making the photocatalytic active layer 13 is todeposit a layer of titanium dioxide on the surface of the carbonnanotube structure 112 by atomic layer deposition. Another method ofmaking the photocatalytic active layer 13 is to sputter or deposit atitanium layer on the surface of the carbon nanotube structure 112.During the sputtering process, oxygen flow is introduced and thetitanium oxide layer is oxidized to obtain titanium dioxide.

The photocatalytic structure 10C has a great absorption of light in thewavelength range from 200 nm to 400 nm, and the absorbance can be up toabout 93%. The inventors of instant application find that thephotocatalytic structure 10C including different intersected drawncarbon nanotube films as carrier can adjust the absorption of thevisible light.

TABLE 3 Different conditions for making photocatalytic structure inembodiments 3A to 3D and comparative examples 4 to 7 Layer number ofintersected stacked drawn Type of Embodiments carbon nanotube filmsdeposited metals 3A 1 Ag 3B 2 Ag 3C 3 Ag 3D 4 Ag Comparative 1 Nodeposited metal example 4 Comparative 2 No deposited metal example 5Comparative 3 No deposited metal example 6 Comparative 4 No depositedmetal example 7

An angle between the aligned directions of the carbon nanotubes in twoadjacent stacked drawn carbon nanotube films is less than 0 degrees andlarger than or equal to 90 degrees. The aligned directions of the carbonnanotubes in any two drawn carbon nanotube films are different. In oneembodiment, the carbon nanotube structure 112 includes two intersectedstacked drawn carbon nanotube films. The angle between the aligneddirections of the carbon nanotubes in the two adjacent stacked drawncarbon nanotube films is about 90 degrees. In another embodiment, thecarbon nanotube structure 112 includes four intersected stacked drawncarbon nanotube films. The angle between the aligned directions of thecarbon nanotubes in any two adjacent stacked drawn carbon nanotube filmsis about 45 degrees and the aligned directions of the carbon nanotubesin any two drawn carbon nanotube films are different from each other.Referring to FIG. 23 and FIG. 24, scanning electron microscope (SEM)images of the carbon nanotube structure 112 including four intersectedstacked drawn carbon nanotube films and the photocatalytic structuremade by embodiment 3D are provided. FIG. 24B is a partially enlargedview of FIG. 24A. The transmission spectra of visible light for thephotocatalytic structures prepared in embodiments 3A-3D and comparativeexamples 4-7 is also provided. Referring to FIG. 25, experimentalresults show that the light absorbance of the photocatalytic structureafter annealing is higher than that of the corresponding photocatalyticstructure without annealing treatment, regardless of the number of thelayers of the intersected stacked drawn carbon nanotube films. Themaximum light absorbance of the photocatalytic structure withoutannealing treatment only occurs at 280 nanometer wavelength. Thephotocatalytic structure deposited metal layer has stronger absorptionin broadband. Larger resonance absorption of the photocatalyticstructure occurs at 360 nanometer wavelength. With the increase of thelayer number of intersected stacked drawn carbon nanotube films, theabsorption ability of the light near 360 nanometer wavelength isenhanced. The maximum absorbance of the photocatalytic structure withfour intersected stacked drawn carbon nanotube films can reach 93%,which indicates that the utilization of ultraviolet light has beenimproved.

The photocatalytic structure 10C as disclosed has the followingcharacteristics. Firstly, it is convenient to recycle the photocatalyticstructure 10C to reduce secondary pollution because the carbon nanotubestructure 112 acts as the carrier. Secondly, photocatalytic reaction canbe carried out on both surfaces of photocatalytic structure 10C, so thespecific surface area of the photocatalytic structure 10C forphotocatalytic reaction can be increased, and the utilization rate andcatalytic effect of the photocatalytic structure 10C can also beimproved. Thirdly, the nanostructure of photocatalytic structure 10C andthe resonance absorption wavelength and absorption intensity of themetal layer 14 to solar light can be controlled by adjusting the numberof layers and the manners of intersecting and stacking of the drawncarbon nanotube films in carbon nanotube structure 112. The absorbanceof ultraviolet light and the catalytic effect of titanium dioxide inultraviolet wavelength can be greatly improved. Fourthly, variousphotocatalytic structures with different characteristics can be preparedby placing the carbon nanotube composite structure on differentsubstrates. When the material of the substrate is metal, thephotocatalytic structure can be used as working electrode ofphotoelectrocatalytic structure. The working electrode, a counterelectrode, and a reference electrode can be assembled into aphotoelectrocatalytic reactor.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the disclosure. Any elements describedin accordance with any embodiments is understood that they can be usedin addition or substituted in other embodiments. Embodiments can also beused together. Variations may be made to the embodiments withoutdeparting from the spirit of the disclosure. The above-describedembodiments illustrate the scope of the disclosure but do not restrictthe scope of the disclosure.

Depending on the embodiment, certain of the steps of methods describedmay be removed, others may be added, and the sequence of steps may bealtered. It is also to be understood that the description and the claimsdrawn to a method may include some indication in reference to certainsteps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

What is claimed is:
 1. A photocatalytic structure comprising: a carbonnanotube structure; a photocatalytic active layer coated on the carbonnanotube structure; and a metal layer comprising a plurality ofnanoparticles on the surface of the photocatalytic active layer; whereinthe carbon nanotube structure comprises a plurality of carbon nanotubesintersected with each other and defines a plurality of openings, and thephotocatalytic active layer is coated on the surface of the plurality ofcarbon nanotubes.
 2. The photocatalytic structure of claim 1, whereinthe carbon nanotube structure comprises a first carbon nanotube film anda second nanotube film stacked and intersected with each other.
 3. Thephotocatalytic structure of claim 2, wherein the first carbon nanotubefilm comprises a plurality of first carbon nanotubes aligned along afirst direction, the second carbon nanotube film comprises a pluralityof second carbon nanotubes aligned along a second direction, an angle isdefined between the first direction and the second direction.
 4. Thephotocatalytic structure of claim 3, wherein the angle between the firstdirection and the second direction is larger than 0 degrees and lessthan or equal to 90 degrees.
 5. The photocatalytic structure of claim 1,wherein the carbon nanotube structure comprises a plurality of carbonnanotube films, carbon nanotubes of each of the plurality of carbonnanotube films are aligned along an alignment direction, and theplurality of carbon nanotube films are arranged so that none of thecarbon nanotube films has a same alignment direction.
 6. Thephotocatalytic structure of claim 1, wherein the metal layer comprises aplurality of nanoparticles on the surface of the photocatalytic activelayer.
 7. The photocatalytic structure of claim 1, wherein the metallayer comprises a material selected from the group consisting of gold,silver, copper, iron, nickel, aluminum, and alloy thereof.
 8. Thephotocatalytic structure of claim 1, wherein the metal layer covers anentire surface of the photocatalytic active layer.
 9. The photocatalyticstructure of claim 1, wherein the metal layer covers part surface of thephotocatalytic active layer.
 10. The photocatalytic structure of claim1, wherein the photocatalytic active layer comprises a material selectedfrom the group consisting of titanium dioxide, zinc oxide, tin oxide,zirconium dioxide, and cadmium sulfide.
 11. A method for making aphotocatalytic structure, the method comprising: providing a carbonnanotube structure comprising a plurality of carbon nanotubesintersected with each other; a plurality of openings being defined bythe plurality of carbon nanotubes; forming a photocatalytic active layeron the surface of the carbon nanotube structure; applying a metal layerpre-form on the surface of the photocatalytic active layer; andannealing the metal layer pre-form.
 12. The method of claim 11, furthercomprising arranging the photocatalytic structure on a substrate afterannealing the metal layer pre-form.
 13. The method of claim 11, whereinthe substrate comprises a material selected from the group consisting ofglass, quartz, silicon, silicon dioxide, silicon nitride, galliumnitride, gallium arsenide, alumina, magnesia, iron, copper, titanium,chromium, aluminum, zinc, polyethylene terephthalate, polyimide,polymethyl methacrylate, polydimethylsiloxane, and polyethylenenaphthalate.
 14. The method of claim 11, wherein the method of formingthe photocatalytic active layer on the surface of the carbon nanotubestructure comprises depositing a layer of titanium dioxide on thesurface of the carbon nanotube structure by atomic layer deposition. 15.The method of claim 11, wherein the method of forming the photocatalyticactive layer on the surface of the carbon nanotube structure comprisessputtering a titanium layer on the surface of the carbon nanotubestructure, and obtaining titanium dioxide by oxidizing the titaniumlayer during the sputtering process.
 16. The method of claim 11, whereinthe method of annealing the metal layer pre-form is performed at anannealing temperature in a range from approximately 200° C. toapproximately 1000° C., and for a time period in a range fromapproximately 10 minutes to approximately 6 hours.
 17. The method ofclaim 11, wherein the method of applying the metal layer pre-form on thesurface of the photocatalytic active layer is performed by electron beamevaporation, ion beam sputtering, atomic layer deposition, magnetronsputtering, thermal vapor deposition, or chemical vapor deposition. 18.The method of claim 11, wherein a thickness of the metal layer pre-formranges from approximately 2 nanometers to approximately 100 nanometers.